feat(input): #89 cursor capture — Input.cursor_mode (hide/lock/confine)

A windowed action game can hide the OS cursor and lock/confine the mouse to the
window. Input.cursor_mode(mode): 0 normal, 1 hidden (draw your own reticle),
2 locked (hidden + dissociated — the mouse feeds relative motion via
Input.mouse_dx/dy and Input.mouse_x/y is a clamped virtual cursor, FPS/twin-stick
aim), 3 confined (dissociated but visible; the mouse can't leave the window).
The platform auto-releases (shows + reconnects) while the window is not key
(Cmd-Tab) and on close, so the cursor is never left captured. Headless it is a
no-op (DCE'd).

Native macOS impl in cocoa.ll: [NSCursor hide]/[unhide] (ref-counted, toggled
only on change so the count stays balanced across focus changes),
CGAssociateMouseAndMouseCursorPosition, and CGGetLastMouseDelta for the relative
virtual cursor, behind a new win_cursor_mode intrinsic. Windowed-only behaviour
(not in the headless golden suite); example compiles headless and links
windowed. Full suite 115/0, fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/library/cursor_capture.ludic

@@ -0,0 +1,24 @@
+# cursor_capture.ludic — cursor capture (#89): a windowed game hides / locks the
+# OS mouse and draws its own reticle from the mouse position. Input.cursor_mode is
+# a no-op headless (it only touches the platform cursor on a windowed build), so
+# this compiles and links both ways; the capture behaviour is windowed-only.
+#
+# Run windowed:  bin/ludicc examples/library/cursor_capture.ludic -o /tmp/aim && /tmp/aim
+program CursorCapture {
+  property Tag { n: int = 0 }
+  model P { Tag }
+
+  @OnStart handler Boot {
+    Input.cursor_mode(2)                 # 2 = locked: hide the OS cursor, feed relative motion
+  }
+
+  handler Draw phase Render {
+    Screen.clear(0x101018)
+    # a reticle drawn at the captured mouse position (a virtual cursor while locked)
+    let mx = Input.mouse_x()
+    let my = Input.mouse_y()
+    Screen.fill_rectangle(mx - 3, my - 1, 7, 2, 0xffcc00)
+    Screen.fill_rectangle(mx - 1, my - 3, 2, 7, 0xffcc00)
+    Screen.show()
+  }
+}